1. Field of the Invention
This invention relates to lamps which operate at high temperatures and have a light source which emits both visible and UV light radiation which is surrounded by a UV absorbing quartz envelope codoped with both ceria and titania. More particularly, this invention relates to lamps comprising a UV absorbing fused quartz envelope codoped with ceria and titania which is at a temperature of at least 500.degree. C. during lamp operation and which encloses a source of light which emits both UV and visible light radiation.
2. Background of the Disclosure
Fused silica or fused quartz as it is also known is used as a light-transmissive, vitreous envelope material for high intensity lamps, such as gas discharge lamps and halogen-incandescent lamps, because of its excellent transmission of visible light and its ability to withstand high operating temperatures of up to about 1100.degree. C. Almost all arc discharge lamps and many high intensity filament lamps, such as tungsten-halogen lamps, emit ultraviolet (UV) radiation which is harmful to human eyes and skin and which also causes fading of fabrics, plastics and paint and yellowing and/or hazing of many types of plastics employed in lamp fixtures and lenses. Fused quartz is an excellent transmitter of UV radiation and therefore provides no shielding against the emission of such radiation by an arc or filament light source enclosed within a lamp envelope made of fused quartz. As a result, lamps have been developed comprising a light source which emits both UV and visible light radiation enclosed within a vitreous envelope of fused quartz or glass containing UV-absorbing materials, or dopants as they are called, so that the lamp envelope will, of itself, absorb the UV radiation emitted by the light source. Illustrative, but non-limiting examples of such efforts in the past are disclosed in U.S. Pat. Nos. 2,895,839; 3,148,300; 3,848,152; 4,307,315 and 4,361,779. However, there is still a need for a vitreous material useful for lamp envelopes which are heated to a temperature above 500.degree. C. during lamp operation and which will absorb UV radiation at wavelengths from 200-380 nm along with minimal absorption of visible light radiation from 380-750 nm. Such a material should also be a homogeneous, colorless, glassy material and dopants present should be of a type and in an amount which minimizes or avoids chemical reactions between the doped lamp envelope and metal halides and other chemicals present in both an arc discharge lamp and a halogen-incandescent lamp. The ability of the material to be used at temperatures in excess of 500.degree. C. should not be impaired by the dopants or the material will not be useful for high temperature lamps.